1. Field of the Invention
The present invention relates to optical communications, and more particularly, to a system and method for collimating and redirecting beams in a fiber optic system.
2. Discussion of the Related Art
The integration of aspheric lenses within connectors designed to couple light beams from one optical fiber to another, and manufacturable using injection molding of optically transparent plastic, is well known. These applications generally address the need to maintain alignment between the axis of an optical fiber and a light beam output therefrom. One group of previous known designs describes the placement of a concentric plano-convex lens with the flat side of the lens near or against a fiber core. Another group of known designs is shown in FIG. 1a. This type utilized the placement of plano-convex lens with the flat side away from a fiber core and an air gap cavity separating the lens and the fiber. Shown in FIG. 1a is a cross-sectional view of a prior optical fiber-connector assembly, comprising the integration of an optical fiber 2 having a fiber core 3 with a fiber connector housing 1. The optical fiber 2 and a collimating lens 7 are positioned so that a cavity 5 is formed between them, with the flat side of the collimating lens 7 being away from the fiber core 3. As an extension to the design shown in FIG. 1a, the design in FIG. 1b illustrates the use of a wedge 6 to redirect the beam at 45° using total internal reflection (TIR) after the beam is collimated by the collimating lens 7.
Similar assemblies designed to couple light directly from a vertical cavity surface emitting laserdiodes (VCSEL) into a multimode fiber and/or couple light from an optical fiber directly onto a photodetector also appear in the prior art. One such assemblies has a design with a concentric TO can ferrule, lens, and fiber ferrule elements. Designs of this nature, applying specifically to coupling between VCSELs and photodiodes mounted inside TO cans and optical fibers, are devised for use in serial data links rather than wavelength division multiplexing (WDM) systems.
More recent designs involving wavelength division multiplexers (WDM) employing thin film filter (TFF) channel separation and a “zig-zag” configuration may be subdivided into two types of designs. The first type of designs centers around the use of optical waveguides, consisting of regions of high index material (core) surrounded by a lower index material (cladding), to route the light along the “zig-zag” waveguides. The second type of designs involves those designs that depend on collimation and free-space “zig-zag” optical routing. In implementation, the collimation, redirection, and focusing of light relevant to the second type of designs, or the free-space “zig-zag” multiplexer/de-multiplexer designs, differ drastically from the first type of designs, or the waveguide-based solutions. Prior art involving TFF-based wavelength division multiplexers (WDM) that employ a free-space “zig-zag” configuration generally applies to fiber-to-fiber applications such as optical switches, branch filters, and add-drop multiplexers. Most of these designs have a planar topology that is not well suited for current injection molding technology. Therefore, there is a need for a system and method that utilize free-space “zig-zag” optical routing while being suited for current injection molding technology.
There is little, if any, prior art that describes a design for a TFF-based optical WDM transceiver that uses injection molding of transparent plastic to construct an integrated optical assembly. One example of a related design was presented by B. Wiedemann at the IEEE 802.3ae Interim Meeting in 2000. The input collimator of this design is consistent with the air-gap cavity design mentioned earlier with respect to FIGS. 1a and 1b. A serious disadvantage of this design and the designs of FIGS. 1a and 1b is the absence of a ferrule to guide the fiber along the axis of a collimating lens. A small shift in fiber position results in a serious misalignment of the collimated beams. If standard injection molding techniques were used to manufacture the design, addition of the ferrule that is necessary to refine the design would be extremely difficult because shaping the lens and ferrule on the same “slide” would generate an undercut condition.
An additional problem is depicted in FIG. 2, which shows that a collimating element of related design consists of a lens surface positioned on a tilted base of refractive material. The diagram suggests that, by design, the chief optical ray of a beam 8 from a point source 4 strikes the surface of a lens 7 near its center and refracts into the refractive material 9 at an angle equal to the tilt angle of the base on which the lens surface is mounted. The tilt angle of the base is used to redirect the chief ray of the beam 8 to the desired angle, while the curvature of the lens 7 is used to collimate the beam 8. Because the chief ray is deliberately designed to penetrate the surface of the lens 7 off its axis of symmetry, the quality of beam collimation is sacrificed. It is impossible to eliminate aberration in the beam 8 even if aspheric terms are added to the sag equation defining the lens 7. Aberration is especially great for sources of large numerical aperture, for large tilt angles, and for sources displaced slightly from the optimal position. Therefore, there is a need for a system and method that collimates and redirects beams in a fiber optic system in a more efficient manner.